Selective circuit arrangement



Jan. 12, 1932. J. HERMAN SELECTIVE CIRCUIT ARRANGEMENT Filed Feb. 5,1929 3 Sheets-Sheet .5 Sectz'om .8 Sectiow Mu wswk mw INVENTOR v JflIlman/ ATTORN EY Jan. 12, 1932. HER AN 1,840,360

SELECTIVE CIRCUIT ARRANGEMENT Filed Feb. 5, 1929 5 Sheets-Sheet 2fimyaedamce 17' .4, wmmweczm,

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5 x E 8 Q INVENTOR JJFermmI/u Jan-12, 1932. J. HERMAN 1,840,360

SELECTIVE CIRCUIT ARRANGEMENT INVENTOR BY Jfirmalv ATTORN EY -cuit inwhich it is connected; In accordance 'transmission from the impedancewithout Patented Jan. 12, 1932 f flj funrrso,sTArEs PATENT OFFICE IJosnrn HERMAN, OF wEsrrIELn, NEW'J'EESEY, ASSIGNOR TO AMERICAN TELEPHONE.AND TELEGRAPH COMPANY, A CQRPOR-ATION on NEW YORK SELECTIVE CIRCUITARRANGEMENT ib ing series and shunt impedance elements,

operate to discriminate between the band or hands of frequencies passedand the band or bands'of frequencies suppressed byreason of theattenuation properties of the filter.

f-These attenuation properties are such that within the band orbands offree transmission, the filter has little or no attenuation, but tends tosuppress*frequencies lying outside the band or bands offrcetransmission. The degree of selection depends upon'the attenuationto whichthc frequencies outside of the free transmission range aresubjected- The 'attenuation in the suppressionrange in turn depends'uponthe number of sections of the Jfi-lter, the, attenuation being greateras the number of sections increases. 7

1 In using a filter in the manner above de scribed its selectivefunctions do not depend upon the impedance looking intotlie filter,

' 5513 and the impedance is only of importance from the standpoint ofconnecting the filter in a circuit in such a way that reflection losseswill not occur due to improper matching of the impedances of the filterof tliecirwithjthe present invention, however, aband selecting action isobtained by taking advantage of the fact that the impedance of thefilter is different within its range of free the range. Thisisaccomplished by intercon- 'Jiecting two sectionsof the circuit inwhich the selective action is to take place through a hybrid coil orother balanced circuit arrangeat with the filter connected on one sideof mentand a suitable network or element connected on the other side tobalance the1mpedance of the filter in the range over which Applicationfiled February 5, 192

i ,'brid-coil otherbalancing arrange- Serial No. 337,-3i4.

circuit. With such an arrangement, if the impedance of the filter in agiven range is balanced by a network whose impedance is the same, orsubstantially thesame, over that range, while the impedances of thefilter and balancing circuit are different for frequencies without therange, the two sections of the transmission circuit will besubstantially conjugate over the range of frequencies for which abalance is obtained but, will not be conjugate for frequencies outsidethe range; Consequently, a selective action will take place, which isdue to the impedance characteristics of the filter and its balancingnetwork.

lVith an arrangement such as above described, itwill generally be truethat where a band pass type of filter is employed, a suppression of thefrequencies corresponding to this band will occur in the transmissioncircuit through the hybrid coil or balancing ar rangement, whileconversely, if a band suppression type of filter is employed, the bandpass range of the transmission will substantially coincide with the bandsuppression range of the filter. In general, the sharpness of theselection and the relative amount of discrimination as betweenfrequencies passed and frequencies suppressed will not require a largenumber of filter sections, since the discrimination depends upon theimpedance balance between the filter and its balancing circuit atvarious frequencies.

The invention may be obviously embodied in a wide variety of forms. Forexample, in some cases, a filter on one side of the hy brid coil may bebalanced by a network consisting of pure resistance on the other sideand a SllfilClBIlt degree of balance and sup prcs"'cn obtained. In otherinstances, a filter of one type will be connected on one i de of thehybrid coil and it may be balanced bv means of a filter of another typeon the other side of the hybrid coil, the two filters ng of suchcharacter that their impedances l be substantially the same withinacertain range where suppression is to occur, but will be quitedifferent in the range where free transmission is to take place.

has the advantage that if the filter circuit be opened orshort-circuited, the selective effect of the filter is eliminated fromthe transmission circuit without opening the transmission circuititself. Furthermore, the filter may be effectively removed from thetransmission circuit in this manner by the use of a single'contact. Thecircuit arrangement also en ables a filter network, which normallyfunctions as a band pass filter, to function as a band suppressionfilter, and vice versa.

The invention will now be more'fully un derstood from the followingdescription, when read in connection with the accompanying drawings, inwhich Figure 1 is a circuit diagram of one embodiment of theinventionfFigs'. 2 and 3 are curves illustratingthe operation of thecircuit of Fig. 1; Figs. 4' and'fi 'are diagrams of filter circuitswhich may be employed in connection with the circuit arrangement of Fig.1; Figs. 4a 'ancloa are impedance curves for the filters cf Figs. 4c and5, respectively; Fig. (iis a curve showing the attenuationcharacteristics of the selective circuit when filters of the type ofFigs. 4 and 5 are employed; Figs. 7

and 8 are circuit diagrams of still further types offilter circuitswhich' may be emplo 'gied in connection with the circuit of Fig. 1;Figsr'iaa'nd 8a are impedance curves for the filters of Figs. 7 and 8.,respectively, and '9 'is a curve showing the attenuation characteristicsof the selective circuit when employing filters of the types of Figs. 7ELIIC 8. l

nected to thermidpoints of the hybrid coil so that when a balance isobtained between terminals 1112 and 13-44 of the hybrid "001i, at anyfrequency, the circuit TL will be conjugate with respect to the circuitTL,

thereby substantially preventing transmission between the two sectionsof the circuit 7 at such frequencies as the condition'of balanceobtains.

' Terminals and 1 1 are associated througha transformer 15 with a filterBF.

The'filter BF is-ai. known type of filter com-- prising two dissimilarsections, the series element of each'section comprising an induct- V V.nt inductance, terinmates-in inid series, that is, the series resonantelement comprising the inductance and capacity has' one -h-alf the valueof the full Y series element of a normal section. The lef hand-section,which includes the shuntcac5 pacity element, terminates in aneight-tenth series section, and the distant end of the filter is closedthrough a resistance R. V

The opposite terminals 11 and 12 of the hybrid coil are connectedthrough a transformer 16 with a balancing network which, in the case ofthe filter disclosed, may be a simple resistance R equal in value to theterminatingresistance R of the filter. By means of a switch 21, ashort-circuit may be established across the input terminals 19 and 20ofthe filter BF to effectively remove the filter from the circuit andunbalance the hybrid coil 12 so that transmission may take place from TLto TL freely. The same result may be obtained by opening the circuit ofthefilter BF through switch 22. In either case, the effectiveelimination of'the band filter BF from the circuit takes place withoutopening or closing'any contacts in thetransmission circuit from TL toTL, and the operation of a single contact only is necessary to removethe'filter.

As is well known, afilter of the type BF may be designed so that a bandof frequencies between its cut-off limits f and 7'; may be transmittedfrom end to end through the filter without substantial attenuation,while frequencies outside-of the cut-off limitswill be attenuated inpassing through the filter. The selective action resulting fromtransmission end to end through thefilter depends upon its attenuationproperties. "Its use in the circuit ofFig. 1, however, depends upon theimpedance characteristics of the filter as v r seen from the terminals19*20.

Referring to Fig. 1, TL and TL desig- 'nate sections of a transmissioncircuit which are interconnected for transmission through. a hybrid coil10, the circuit TL being con- 'Theimpedance characteristics'of thefilter are illustrated by the curves of Fig. 2, in which 1" is a curveshowing the variation of the resistance component of the impedance withfrequency, and the curve a" shows the variation of the'reactancecomponent of the impedance with frequency. As will be noted, theresistance r is substantially negligible in the range outside the passband of the filter which extends from frequency f to frequency fl/Vithin the range of the pass hand, however, the resistance componentis equal to the terminal resistance R near the middle of the band andgradually decreases towards zero as the limiting-frequencies f and f areapproached; due tothe dissipation effect of the filter." The. reactance,as shown by the curve as, is zero near the middleof the band andgradually becomes more and more negative as the frequency approacheszero, and more and more positive in value as the frequency approachesinfinity. The curve B of Fig. 2-represents the constant'resistancecomponent of the impedance of the balancing element R of Fig. 1. Wherethe balancing element is a. pure resistance, as illustrated in Fig. 1,it will, of course, have no reactance component. 7 V I 1 It will beobvious from'a consideration of thecurves of Fig; 2 that near the middleofthe band where the react'anc'e components 'iszero and the resistancecomponent 2* is V nent of the filter BF, with the result that theattenuation, as shown by the curve of Fig. 3, dro s, very rapidly, sothat itapproches a minimum atfthe limiting frequencies f and At zerofrequency and at infinite frequency, a condition of'complete unbalanceoccurs because at these frequencies the resistance component 1' of thefilter is zeroes compared'with' the finite resistance R of the balancingcircuit. the same time, the reactance 'of the filter element is infiniteat zero frequency and at infinite frequency. The

result;is that for transmission from TL to, TL there 1s practicallycomplete suppres sion of the band of frequencies near the middle-of thepass band of the filter which lies between the frequencies 7' and Thesup.- pression band resulting from the use of the filter in thismanneifliain this instance, narrowervthanthe pass band of freetransmissionhof tlie filter when used normally, due

- to that-the resistancecurve 9 is section.

rounded off nearithe limitingfrequencies of ,thegband, and asubstantial.amountcf reactance is alsopresent near the edges of thehanrh. resulting in a considerable degree of unbalance before the.limiting frequencies f and flarereached. e

A- somewhat better and more sharply :defined suppression bandcharacteristic may be obtained by substituting for the; filter BF inFig, 1 a band filter section of the type BF of Fig. 4 and bysubstituting for the balanc ing resistance R a band filter section ofthe type illustrated at BF, of Fig. 5. in this case, theterminals 19 and20 of the filter BF will be connected to terminals 19 and 2001i Fig. '1,while the terminals 17 and 18 of the filter BF wil be connected to theterminals 1?. h11d 18 of Fig.v 1.1 It will, of course, obvious thatthetwo filters BF and BF, .may beinterchanged in Fig. 1 without chang-.,ing the result; The filter BF comprises a single-section 't/h 3 I i/type illustrated on page lof an article entitled-Theory and design ofuniform and -,composite electric wave-filters out J. Zobel, published inthe Bell System Technical Journal for Januaryl923,nbeginning at 10fVolume II,

o. 1. .This filtorfis-oftheband pass type, .and, as herein used,terminates in m d-shunt "In rder to understand what. is meant by "the'e'zrpi'fession mid shunt termination, it

should be noted that the inductance and capacities of the series andshunt elements of filter BF are given the same designations as in theaforementioned article by Zobel. If we compare the filter section ofFig. 4 with the filter section shown at IV on page 41 of the-Zobelarticle, it will be seen that the series element in F l has the samevalues as the series element in the Zobel filter. Now each shuntresonant element in the Zobel filter, such as, for example, L C may beconsidered as being made up of two parallel elements having twice theimpedance of the single shunt resonant element, or, in other words, oftwo shunt resonant elements each having inductance 2L and capacity If,now, one-half of each of the shunt resonant circuits of the Zobel filtersection as thus constituted is eliminated,- the filter section isterminated in amid shunt section.

If, further, the eliminated half shunt impedances are connected on theopposite side of the series element, we have the arrangement shown inFig. 4 of this application, with the filter section terminated at bothends in mid shunt. In other words, the filter BF of Figs lSktCOIHPlBtQfilter section equivalent in all respects to the filter IV of the Zobelarticle, differing only in the fact that the shunt impedance element ofthe filter is divided into two portions and arranged one ateither sideof the series element. A pure resistance R ill be connected across thedistant end of the filter section BF as shown. I The variation of theresistance and reactance components of the impedance of the filtersection il -F as seen from the terminals 19-20, are shown in Fig. la. Inthis figure, the curve 1 is the variation of the resistance component ofthe filter with frequency, while tl e curve a: is the variation of thereactance component with frequency. The straightline curve R representsthe constant value of the terminating resistance R of the filter for allfrequencies.

The filter section at Fig. 5 is of the type 1V shown on page 4-1 of theZobel article above mentioned. The filter section of Fig. 5 isterminated in mid series. In other words, if the series elemei'at of thefilter section on page 41 of the Zobel article be divided into twohalves, arranged in series with each other and one on either side of theshunt element, we will have a complete filter section terminated at eachend in mid series, as illustrated in Fig. 5. If now, the filter sectionof Fig. 5 be terminated by a resistance R at its far end, the impedancelooking into the terminals 1718 will be as illustrated in Fig. 5a. Inthis figure, the curve 1" represents the variation of the resistancecomponent of the impedance with frequency, and the dotted line curve 00the variation of the reactanee component with frequency, thestraight-line curve B, as before, representing the'constant value of theterminating resistance R of Fig. 5. Let us nowcompare the impedance ofthe lter section of Fig. 4, as shown in'Fig. 4a, with the impedance ofthe filter section of Fig. 5' as shown in Fig. 5a. The frequencies f andf may be taken as the limiting frequencies of the bands of freetransmission, while the frequencies f and f are the frequencies oneither side of the band of free transmission at which the attenuation isinfinite. If we examine the resistance component of the impedance ofthefilter of Fig. 4, it will be seen that it closely approximates the-valueof the terminating impedance R within the band represented by thelimiting frequencies f and l gand is approximately zero-outside thezbandof'free transmission. The resistance is actually zero at frequencies O,f,f ,,and atinfinite frequency. 'Near the middle of the bandat frequencythe resistance component is just equal to the and does not departgreatly from zero tween frequencies f and f It 1s also zero at frequencyf and fz terminating resistance R, rising suddenly at the limitingfrequencies f and to a value somewhat greater than B. At frequencies f1and 7% within the band'and near the limiting hand, is zero atfrequencies f f and f 1]2 V Y but approaches aninfinite value as thefrequency decreases below f while approaching a positive infinite valueas the frequency increases above I 'If we now examme the components ofthe impedance of the filter section of Fig. 5 as shown in Fig; 5a, wefind that at frequenand 7%, the resistance component is just equaltothegvalue of the terminating resistance .R of the filter, and that itdoes not depart greatly from the value of R in. this range. 'Theresistance component drops off'to zero at frequencies f and j and it isalso zero at zero frequency and at infinite frequency. The reactanee, onthe other hand, is zero at zero frequency graduallyrisesto an infinitepositive value at f and gradually decreases from an infinite negativevalue to zero as frequency f is approached. At frequencies f Thus itwill be seen that at frequencies f and f the resistance components ofboth filter sections are equal to the value of the terminatingresistance R and the reactancecomponents in both cases are zero, so thatat these frequenciesa perfect balance is obtainedv and infiniteattenuation results in transmission from'TL to TL. At zero'frelquency,the resistance components are both equal and have zero value, but ,acomplete condition of unbalance exists by reason of the factthat thereactanee component is, in the case of the filter of Fig. 5',zero,'while it is infinitely negative in the case of the filter of Fig.4:. At zero frequency, therefore, a condition of no attenuation wouldexist for transmission from TL to TL. The same holds true for"frequencies f and j for at these frequeneies,while the resistancecomponents are equal to each other since they are both zero, thereactanee component of the filter section of Fig. 4 is zero, while thatof the filter section of Fig. 5 is infinite, so

that a complete unbalance exists. So also, at

infinite frequency, a condition of complete unbalance existsnotwithstanding the fact that both resistance components have zerovalue. the imbalance being due to the fact that the reactanee componentin the one case is zero, and in the other case is infinite.

The completecurve showing the'variation V of attenuation with frequencyfor transmission from TL to TL, using the filter sections I of Fig. 4and Fig. 5, is illustrated in Fig. '6.

'It will be seen from this curve that the band pass filters of Figs. 4andf5, when'connected in a balanced circuit, give .a band suppressioncharacteristic having very sharp cut-off as the cut-oiffrequencies f andf are approached, the attenuation being very small in the region oneither side of the suppressed band, and being infinite at three pointswith in thesuppressed band while being very large atall other pointsWithin the band between the frequencies f, and f2.

. the pure resistance such as R of Fig. 1 having a value equal to theterminating resistance R of the network. For example, consider thenetwork of Fig. 5 and its impedance characteristics as shown in Fig. 5a.The balancing resistance B would then have an impedance such asindicated by the straight-line curve R of Fig. 5a. .with. the impedancecurves 2" and an, it is Comparing this impedance evident that therewould be an exact balance at frequencies f /f f and f It is also evidentthat in the range from zero to f and from 7% to infinity, the reactancewould be unbalanced. Likewise, the resistance components would beunbalanced in the ranges outside of the cut-off frequencies of thefilter. The result is that for transmission from TL to TL theattenuation frequency curve would be of substantially the same shape asthat indicated by Fig. 6.

A band passv characteristic for the circuit TL TL may be obtained byconnecting "band suppression filters'of the types shown in Fig. '4' andFig. 8, one on either side of the hybrid coil. The filter section ofFig. 7 is of j the type 1H shown on page 40 of the Zobel article abovereferred to, the filter section in this case being terminated on eachside in the mid shunt section witha resistance R connected across itsdistant termination.

f Likewise the filter section of Fig. 8 is of the type TEL on page 40 ofthe aforesaid Zobel T article, this filter section being terminated atJ1... i iTfi and f...,, all of which lie.

each side in mid series with a resistance R' connected across itsdistant termination.

The. impedance components of the two filter sections are shown in Figs.7a and 8a,

respectively. Comparing the curves of these two figures. it will be seenthat at frequencies 7, withinthe suppression band common to the twofilters, the resistance components are both equal to each other andequal to zero,

but the-reactance components at each 'ofthese p frequencies is zero inthe one case and infinity in the other, so that a condit on of completecircuit TL to circuit TLis zero.

unbalance exists, and the attenuation from At frequencies f and f lyingjust outside the limiting frequencies f and f of the suppression band ofthe two filters, the resistance components in both cases are equal tothe value: of the terminating resistance R, and

the 'reactance components in both cases are zero, so that perfectbalanceresults and the attenuation from circuit TL to circuit TL isinfinite.The same condition occurs at zero frequency and at infinite frequency.

The complete attenuation frequency char-- acteristic is indicated by thecurve of Fig. 9. From this curve, it will be seen that within thesuppression band of the two filters (which lies between frequencies fand f a condition of substantially zero attenuation exists fortransmission from circuit TL to circuit TL, while in the frequencyranges on either side of the limiting frequencies f and asuppressionrange occurs, each suppression 'ange being bounded by pointswherethe suppression is infinite with a range of value equal to theterminating impedance R of the filter. For example, the filter of Fig.8, when balanced by a pure resistance, would give accurate balance atfrequencies 0 and f and a fair degree of balance at interveningfrequencies. Accurate balance would also occur at frequency f and atinfinite frequency witha fair degree of balance in tlie'interveningrange. In the suppression range of a filter, the-reactance component, asindicated in Fig. 8a, would not be balanced, while the resistancecomponent of the, filter would either be very small or actually zeroascompared with the finite resistance R of the balancing element. Theresult is that the attenuation frequency curve for-transmission from TL.plete unbalance may be brought about at all frequencies byshort-circuiting the input terminals of the one filter andope'n-circuiting the other filter, so that free transmission "takesplace at all frequencies from TL to TL.

Likewise, a condition of perfect balance at all frequencies may bebrought about by shortcircuiting the input terminals of both filters orby open-circuiting both filters, in which case transmission from TL toTL will be prevented at all frequencies.

It will also be obvious that various other types of filters andcorresponding balancing networks may be employed in connection with thehybrid coil arrangement to produce other types of. band suppression andband'transmission effects, and that the genthe other two, a transmissioncircuit having two sections associated Wltll said last two points, anetwork associatedwith one of the balancing pointsof saidbalancingelement,

said network having substantially uniform impedance in one range offrequencies and a materially diife'rent impedance outside said range,andan impedance device associated with the other balancing point of saidbalancing element in balancing relation to sa d network, said impedancedevice having substantially the same impedance as said neti work in saidfirst mentioned range of frejquencies and a different impedance outsidesaid range, whereby said balancing element is'balanced to preventtransmission of said firstmentioned range of frequencies over saidftrai'ismission circuit, but is unbalanced to perunit transmission offrequencies outside said range.

2. In a selective syste na balancing element having four connecting ponts so arranged that when two of said points are balance'd notransmission takes place between the other two, a transmission circuithaving jf two sections associated with aid last two points, a networkassociated with one of the ba lancing points of said balancing element,said network having substantially uniform impedance in one range offrequencies and a materially different impedance outside said range, andan in'ipedaii'ce device associated with the other balancing point ofsaidbalancing element in balancing relation to said network, saidimpedance device havin a substantially uniform impedance at allrequencies which is equal to the impedance of said netwoi in said firstmentioned range of frequencies, whereby said balancing element isbalanced to prevent transmission of "f said first range of frequenciesover said. transmission circuit, but is unbalanced to permittransmission of frequencies outside; 'sa'idrange,

, UL I 3. In a selective system a balancing ele-' im ent having fourconnecting points so arf ranged that when two of said points arebalanced no transmission takes place between I the other two, atransmission circuit having I two sections associatedwith' said last twopo nts, a network associated with one of the I "balancing points of saidbalancing element, i said network havin two frequency ranges in one ofwhich its impedance is materially sion in the other range.

e se-em from that of "the other, an impedance device associated with theother balancng point of said balancing elei'ne'nt'in' balancing relationto saidnetwork, the impedance of saidfldevice being substantially the'same as that of said network over'one frequency range but materiallydifferent in the other range, whereby said balancing element is balancedto prevent transmission of one range'offrequencies over saidtransmission circuit and is unbalanced to permit transmisalpIn aselective system a balancing element having four connecting points soarranged that whentwo of said points are balanced no transmission takesplace between the other'two, a transmission circuit having two sectionsassociated with said last two points, a band filter associated with oneof the balancing points'of said balancing element and having anattenuating range and a range of free transmission, said filter havingsubstantially uniform impedance in one range and a materialiydiiierentimpedance in the other range, and an impedance device associted with theotherbalancing point of said balancingelement in balancing relation tosaid filter, said impedance device having substantially the sameimpedance as said filter in a range corresponding to one of the rangesof said filter andia different impedance outside said range, wherebysaid balancing element is'balanced to prevent transmission of one ofsaid frequency ran es over said transmission circuit, but is unbalancedto permit transmission of frequencies outside said range.

In a selective system a balancing ele ment having four connecting pointsso arranged that when two of said points are bal aiiced no transmissiontakes place between the other two, a transmission circuit having twosections associated with said last two points, a band filter associatedwith one of the balancing points of said balancing ele-* ment and havingan attenuating range and a range of free transmission, said filter having substantially uniform impedance in one range and a materiallydifferent impedance in tlie'other range, and an impedance device"associated with the other balancing point of said balancing element inbalancing relation to said filter, said impedance device having asubstantially uniform'impedance at all frequencies which is equal to theimpedance of said filter in one, of its frequency" ranges, whereby saidbalancing element is balanced to prevent transmission of one range offrequencies over said transmission circuit but is -unbalanced to permittransmission of frequencies outside of said range.

6. In a selective system a balancing element having four connectingpoints so arranged that when two of said points are balanced notransmission takes place between "the other two, a transmissionci"transmission in the other range.

*"iit having last two two sections associated with's points, a bandfilter associatedwith one of the balancin -Joints ofsaid balancin elev cment and having an attenuating range and a range of free transmission;the impedance of said filter being materiallydifferent in the'two'ranges, an impedance device associated -with the other balancing'pont of said balancing element in balancing'relation to said filter, theimpedance of said device being substantially the same as that O'f'SllClfilter over one frequency range but'materially different "in the otherrange, wherebysaid balancing of'one range of frequencies over saidtranselement is balanced to prevent transmission mission circuit but isunbalanced to permit 7. In a selective system, a hybrid coil havingbalancing points, a transmission circuit comprising two sections, one ofwhich is associated with a winding of said coil and arranged to transmitthrough the coil to the other section which is associated with themidpoint of another winding of said coil, a network associated with onebalancing point of said hybrid coil, said network having 7 substantiallyuniform impedance in one range of frequencies and a materially differentimpedance outside said range, and an impedance device associated withanother balancing point of said hybrid coil, said impedance devicehaving substantially the same impedance as said network in said firstmentioned range of frequencies and a different impedance outside saidrange, whereby said hybrid coil circuit is balanced to preventtransmission of said first mentioned range of frequencies from one ofsaid transmission sections to the other, but is unbalanced to permitsuch transmission of frequencies outside said range. v

8. In a selective system, a hybrid coil having balancing points, atransmission circuit comprising two sections, one of which is associatedwith the winding of said coil and arranged to transmi' through the coilto theother section which is associated with the midpoint of anotherwinding of said coil, a network associated with one balancing point ofsaid hybrid coil, said network having substantially uniform impedance inone range of frequencies and a materially different impedance outsidesaid range, and an impedance device associated with another balancingpoint of said hybrid coil, said impedance deance at all frequencieswhich is equal to the impedance of said network in said firstmencomprisin g two sections, one of which is associated with the windingof said 0011 and arranged to transmit through the coil to the othersection which is associated with the midpoint of another winding of saidcoil, a

network associated with one balancing point of said hybrid coil, saidnetwork having two frequency ranges in one of which its im edancematerially different from that 0 the other, an impedance deviceassociated with another balancing point of said hybrid coil, theimpedance of said device being substantially the same as that of saidnetwork over on frequency range but materially different in the otherfrequency range, whereby said hybrid coil is balanced to preventtransmission of one range of frequencies from one of said transmissionsections through the hybrid coil to the other, but is unbalanced topermit such transmission in the other range.

10. In a selective system, a hybrid coil having balancing points, atransmission circuit comprising two sections, one of which is associatedwith the winding of said coil and arranged to transmit through the coilto the other section which is associated with the midpoint of anotherwinding of said coil, a band filter associated with one balancing pointof said hybrid coil and having an attenuating range and a range of freetransmission, said filter having substantially uniform impedance in oneof said ranges of frequencies and a materially different impedance inthe other range, and an impedance device associated with anotherbalancing point of said hybrid coil, said impedance device havingsubstantially the same impedance as said filter in a range correspondingto one of the ranges of said filter and a diiferent impedance in theother range, whereby said hybrid coil is balanced to preventtransmission of frequencies in one of said ranges from one of saidtransmission sections through the hybrid coil to the other, but isunbalanced to permit such transmission of frequencies in the otherrange.

11. In a selective system, a hybrid coil havother section which isassociated with the midpoint of another winding of said coil, a bandfilter associated with one balancing point of said hybrid coil andhaving an attenuating range and a range of free transmission, saidfilter having substantially uni;

form impedance in one of said ranges of frequencies and a materiallydifferent impedance in the other range, and an impedance deviceassociated with another balancing point of said hybrid coil, saidimpedance de; vice having substantially uniform impedance at allfrequencies which is equal to the impedance of said filter in one of itsfrequency ranges, whereby said hybrid coil is balanced to preventtransmission of oneof said frequency ranges from one of said trans-:inission sections through theihybrid coil to the other, but isunbalanced to permitsuch Qtransmission' of the other range offrequenoles. v

- point of said hybrid coil and having an attenuating range and a rangeof free transmission,.said, filter having an impedance in one of saidranges which is materially different fromits impedance in the otherrange,

an impedance device associated with an-V other balancing point of saidhybrid coil, the impedanceot' said device being substantially the'sameas that of said filter over a frequency range corresponding to one ofthe ranges of said filter, whereby said hybrid coil is balanced toprevent transmission of one range of frequencies from one of saidtransmission sections through the hybrid coil to the other, but isunbalanced to permit such transmission in the other range.

V 13. A selective system, a hybrid coll having two balancing points, atransmission circult comprising two sections, one of which is associatedwith the winding of said coil and arranged to transmit through the coilto the other section which is associated with the IIllClPOlIlh ofanother winding of said co1l,nn- .pedance elements associated with thebalanci-ng points of said hybrid coil and having such impedance relativeto each other as to produ'ce-a substantial balance over one band offrequencies while being unbalanced for fre Qquencies outside the band.

' 14. In a selective system, a hybrid coil having balancing points, atransmission circuit comprising two sections, one of which is associatedwith the winding of said coil and arranged to transmit through the coiltothe other section which is associated with the midpoint of anotherwinding of said coil, a

network inductively connected to one balancing point of said hybridcoil, and an impedance device inductively connected to an a otherbalancing point of said hybrid coil, said" network and said impedancedevice having a such relative impedances as to produce a balance atfrequencies within a band while bemg unbalanced for a range offrequencies lying outside the band.

15. In a selectlve system, a hybrid coil'having balancing points, atransmission circuit 55 comprising two sections, one of which is.assoruary, 1929-,

ciated with the winding of said coil and arranged to transmit throughthe coil to the other section which is associated with the midpoint ofanother winding of said coil, a

networkinductively connected to one balancing point of. said hybridcoil, an impedance device inductively connected to another balancingpoint of said hybrid coil, said neti work and said impedance devicehaving such relative impedances as to produce a balance at frequencieswithin a band while being unbalanced for a range of frequencies lyingoutside the band, and means to effectively remove the impedance of saidnetwork from thelcircuit without interrupting transmission from one ofsaid transmission sections through the hybrid coil to the other.

In testimony whereof, I have signed my name to this specification this4th day of Feb- JOSEPH HERMAN.

